1. Field of the Invention
The present invention relates to a fan control system capable of modulating a rotational speed of a fan and a method thereof, and more specifically, to a fan control system capable of modulating a rotational speed of a fan to prevent specific resonance frequency and the method thereof.
2. Description of the Prior Art
Electronics technology changes on a daily basis with new advances, so functions of consumer electronic products get more and more diversified, and requests for efficiency get higher and higher. However, high efficiency means high energy-consuming, and it may cause problems of resonance, noise, and heat dissipating. If heat generated by inner components of electronic products can not be dissipated efficiently, it will affect stability and efficiency of operation of electronic products. It even may result in mechanical malfunction cause damage. The conventional solution is to dispose fans on heat sources to decrease temperature thereof, so as to make electronic products work normally.
A pulse width modulation (PWM) fan is usually used in all kinds of computer systems for dissipating heat generated by inner components. The feature of the PWM fan is its linear modulation from low rotational speed to high rotational speed. Taiwan Patent No. 494184 discloses a fan control mechanism of utilizing a single chip to control the rotational speed of the fan by linearly modulation. Please refer to FIG. 1, FIG. 1 is a diagram showing relationship between a rotational speed of a PWM fan and a duty cycle of a PWM signal in the prior art. The modulation of the rotational speed of the PWM fan is according to the operation curve of the rotational speed of the fan as shown in FIG. 1. The operation curve of the rotational speed of the fan integrates the measured result of system noise and heat transfer, and complies with the customer's specification so as to generate the optimal operation curve of the rotational speed of the fan. The optimal operation curve of the rotational speed of the fan can be stored in a basic input/output system (BIOS). A temperature detecting unit is for detecting temperature of a CPU or a key component so as to generate a temperature detecting signal to the BIOS. Afterwards, the BIOS generates the signal of the rotational speed according to the temperature detecting signal to the PWM control chip of the fan, and the fan responses a correct rotational speed. By modulating the duty cycle of the PWM signal, an analog signal is encoded, and the current is added on an analog loading with repeating pulse sequence of on and off. For example, 12V DC power supply becomes a step-down power voltage 3.6V via PWM modulation of 30% duty cycle. Accordingly, different duty cycles are corresponding to different rotational speeds.
Please refer to FIG. 2. FIG. 2 is a relationship table of temperature detecting signals, rotational speeds, and duty cycles of the PWM signal in the prior art. Since the rotational speeds are linear with temperature detecting signals and duty cycles of the PWM signal of FIG. 1 and FIG. 2, each temperature point is corresponding to a fixed rotational speed. However, the operational rotational speeds of the fan cover a very wide distribution range, it may cause resonance due to overlap between vibration frequency of the fan in some specific rotational speeds and natural frequency of a computer casing or of vibration frequency of the fan coupling with the rotational frequency of a hard disk. If resonance between the fan and the casing occurs, it may result in the following problems. It may cause unpleasant resonance sound. Acoustic pressure, acoustic power, acoustic quality may fail to qualify the test specification. It may increase manufacture cost due to structural modification of the casing or changing natural frequency of the casing to improve resonance. It may need modifying mold or remolding due to modification of the casing, and it results in postponement of mass production. If resonance between the fan and the hard disk occurs, it may result in the following problems. It may decrease reading/writing rate of a head of the hard disk, and the head of the hard disk may even be unable to write and read so that the reading/writing rate for the head failing to qualify the test specification. Furthermore, service life of the hard disk may be reduced. In conclusion, it is an important issue to design a fan control mechanism capable of modulating the rotational speed of the fan to prevent specific resonance frequency.